/***Farhan132***/
//#pragma GCC optimize("Ofast")
//#pragma GCC target("sse,sse2,sse3,ssse3,sse4,popcnt,abm,mmx,avx,avx2,fma")
//#pragma GCC optimization ("unroll-loops")
#include <bits/stdc++.h>
using namespace std;
typedef long long ll;
typedef unsigned long long ull;
typedef double dd;
typedef vector<ll> vll;
typedef pair<ll , ll> ii;
typedef vector< ii > vii;
typedef pair < pair < ll , ll > , pair < ll , ll > > cm;
#define ff first
#define ss second
#define pb push_back
#define in insert
#define f0(b) for(int i=0;i<(b);i++)
#define f00(b) for(int j=0;j<(b);j++)
#define f1(b) for(int i=1;i<=(b);i++)
#define f11(b) for(int j=1;j<=(b);j++)
#define f2(a,b) for(int i=(a);i<=(b);i++)
#define f22(a,b) for(int j=(a);j<=(b);j++)
#define sf(a) scanf("%lld",&a)
#define sff(a,b) scanf("%lld %lld", &a , &b)
#define pf(a) printf("%lld\n",a)
#define pff(a,b) printf("%lld %lld\n", a , b)
#define PI 3.14159265359
#define bug printf("**!\n")
#define mem(a , b) memset(a, b ,sizeof(a))
#define front_zero(n) __builtin_clzll(n)
#define back_zero(n) __builtin_ctzll(n)
#define total_one(n) __builtin_popcountll(n)
#define clean fflush(stdout)
const ll mod = (ll) 998244353;
const ll maxn = (ll)2e5 + 5;
const int nnn = 1048590;
const int inf = numeric_limits<int>::max()-1;
//const ll INF = numeric_limits<ll>::max()-1;
const ll INF = 1e18;
ll dx[]={0,1,0,-1};
ll dy[]={1,0,-1,0};
ll dxx[]={0,1,0,-1,1,1,-1,-1};
ll dyy[]={1,0,-1,0,1,-1,1,-1};
bool USACO = 0;
mt19937 rng(chrono::system_clock::now().time_since_epoch().count());
#include <ext/pb_ds/assoc_container.hpp>
#include <ext/pb_ds/tree_policy.hpp>
using namespace __gnu_pbds;
#define ordered_set tree<ll, null_type,less<ll >, rb_tree_tag,tree_order_statistics_node_update>
const ll N = 2e5 + 5;
ll a[N];
ll dp[3][N];
ll n , k;
ll gimme(char c){
if(c == 'J') return 0;
if(c == 'O') return 1;
return 2;
}
ll get(ll id, ll i){
ll lo = i , hg = n;
ll ans = n + 1;
while(lo <= hg){
ll mid = (lo + hg)/2;
if(dp[id][mid] - dp[id][i - 1] >= k) ans = mid , hg = mid - 1;
else lo = mid + 1;
}
return ans;
}
void solve(){
cin >> n >> k;
mem(dp, 0);
for(ll i = 1; i <= n; i++){
char c;
cin >> c;
ll a = gimme(c);
f00(3) dp[j][i] = dp[j][i - 1];
dp[a][i]++;
}
ll ans = n + 1;
for(ll i = 1; i <= n-2; i++){
ll cur = i;
f00(3){
cur = get(j , cur);
if(cur > n) break;
}
if(cur > n) break;
ans = min(ans, cur - i + 1 - 3 * k );
}
if(ans == n + 1) ans = -1;
cout << ans;
return;
}
int main() {
#ifdef LOCAL
freopen("in", "r", stdin);
freopen("out", "w", stdout);
#else
if(USACO){
freopen("balancing.in", "r", stdin);
freopen("balancing.out", "w", stdout);
}else{
ios_base::sync_with_stdio(0); cin.tie(NULL); cout.tie(NULL);
}
#endif
ll T = 1;
//cin >> T;
ll CT = 0;
while(T--){
// cout << "Case #" << ++CT <<": " ;
solve();
}
return 0;
}
